In most air conditioning applications, the cooling load imposed on the system varies over a relatively wide range as outdoor ambient conditions change. To operate a cooling system efficiently, its capacity should be controlled in direct proportion to changes in the temperature conditioning load. Selectively energizing compressors in an air conditioning system having a plurality of parallel connected compressors is one method of varying system capacity to match the cooling load. This method is typically much less expensive than other alternatives, such as using a variable speed drive for the compressor.
Unfortunately, operation of compressors in parallel is known to create certain problems, particularly when one or more of the compressors is de-energized while other compressors continue to operate. One of the most significant problems is the migration of oil in the system to the oil sump of the compressor having the lowest crankcase pressure. That compressor becomes flooded with oil while the other compressors are starved for adequate lubrication, which in an extreme case, may result in their premature failure.
A further problem can arise in compressors, such as rotary or scroll types, which do not have discharge valves to prevent reverse refrigerant flow through the compressor. When such compressors are de-energized, fluid at discharge pressure may flow back through the de-energized compressor from the system discharge manifold to the system suction manifold, substantially reducing the efficiency of the operating compressors. A check valve in the discharge line of each compressor can prevent back flow of discharge fluid, but does not by itself prevent unequal oil levels in the compressors resulting from unequal sump pressures.
If separate oil sump level and pressure equalizing lines are provided to interconnect the oil sumps of each compressor in a multiple compressor system, a portion of the suction gas flowing through the system's common suction manifold diverts through the suction port of an inactive compressor and then into the operating compressors through the pressure equalizing line, reducing the flow of cooling suction gas through the motors of the operating compressors in the system.
Even a small pressure differential between the oil sumps of parallel connected compressors, caused for example by pressure drop due to fluid flow in a pressure equalizing line may cause oil level problems. One p.s.i. of differential pressure is approximately equal to the head pressure of a 30" oil column. Thus, a relatively small pressure differential in the oil sumps, e.g. 0.25 p.s.i., may force a substantial quantity of oil out of a compressor with a higher sump pressure, possibly flooding a compressor with lower sump pressure.
In attempting to solve the lubricant distribution problem in parallel connected compressors, valves have been added to the oil level equalizing line to control oil flow between the oil sumps. For example, U.S. Pat. No. 2,294,552 discloses a float valve disposed in an oil equalizing line, and in a second embodiment, further discloses a valve actuated by differential pressure between the crankcases of the interconnected compressors. Both embodiments allow oil to flow between the crankcases of two compressors until a level in one reaches a minimum safe level, at which point, the valve closes to prevent any further lubricant flow and to prevent refrigerant fluid from flowing through the oil equalizing line.
One drawback of the lubricant handling scheme of the U.S. Pat. No. 2,294,552 patent is that the oil level in each of the two connected compressors varies between a minimum level and a maximum level that exceeds the "normal" level by an amount equal to the difference between the normal and the minimum levels. If three compressors were connected in parallel using this approach, the oil level in any one of the compressors might vary from a minimum level, to a maximum level that is a two times this difference above normal level. This would require an excessively large oil sump in each compressor to accommodate the volumetric changes in the lubricant level while providing adequate lubrication under all operating conditions. Clearly, the above noted scheme is not well suited for use on a system having more than two compressors connected in parallel. In addition, it is not well suited to an application wherein compressors of substantially different size are selectively operated in a parallel system, because the smaller compressor oil sump might not be capable of holding all the oil received from the larger compressor.
In consideration of the problems noted above, it is an object of the present invention to maintain adequate lubricant levels in the oil sumps of a plurality of parallel connected compressors.
A further object of the invention is to prevent fluid flow through an inactive compressor.
Yet a further object is to isolate a compressor that is inactive from the system suction and discharge manifolds.
Still a further object of the invention is to equalize pressure and oil levels in the oil sumps of the compressors while avoiding flow of suction gas through the oil sump of any inactive compressor.
These and other objects of the invention will be apparent from reference to the attached drawings and the description of the preferred embodiment that follows hereinbelow.